1. Field of the Invention
The subject invention relates to circuitry for energizing the printhead of an electrothermal printer.
2. Statement Regarding the Art
One class of thermal printers utilizes a ribbon that generates localized heat internally in response to electrical signals. The localized heat then serves to cause marks to be formed on a receiving medium. Typically, the electrical signals are applied by printhead electrodes wiping across an outer layer of the ribbon that is characterized by a moderate resistivity. These signals migrate inwardly to a layer that is highly conductive (preferably an aluminum layer) with localized heating occurring in the process. The path for the signals is completed by a contact engaging the conducting layer (see, e.g. U.S. Pat. No. 2,713,822) or, alternatively, is completed through the moderately conducting layer at a collection plate (see, e.g. U.S. Pat. No. 3,744,611) where electrical contact is established.
With this type of printer, the signals provided at the electrodes of the printhead cause heating within the ribbon which, in turn, results in a mark being formed. The mark may be produced because of a thermal sensitivity of the paper itself or, as is also known, by a transfer of a portion of an outer thermally transferrable ink layer of the ribbon.
With such "resistive ribbon" printers, print quality has shown undesirable variation when the electrodes are driven by selectively applying a fixed voltage.
It has been found, however, that by using selectively-triggerable current sources to drive the respective printhead electrodes, a satisfactory quality of mark formation may be achieved (see IBM Technical Disclosure Bulletin, Volume 22, No. 2, pp. 790-791).
A shortcoming of the constant-current approach to driving the printhead electrodes arises because individual gated drive circuits are provided for each electrode thereby increasing overall drive circuit complexity and energy consumption.
Indeed, since the current drivers are regulating, rather than merely switching, considerable energy is dissipated making a low cost miniaturized implementation, say in the form of an integrated circuit chip difficult because of cooling requirements.